Undesirable airborne compounds, including sulfur compounds, ammonia, formaldehyde, urea, carbon monoxide, oxides of nitrogen, mercaptans, amines, and ethylene, occur in a number of environments, where most primarily are responsible for the presence of disagreeable odors, or irritating or toxic gases. Such environments include petroleum storage areas, sewage treatment facilities, hospital morgues, animal rooms, and pulp and paper production sites, among others. These undesirable compounds may be bacterial breakdown products of higher organic compounds, or by-products of industrial processes.
Hydrogen sulfide (H.sub.2 S), a colorless, toxic gas with a characteristic odor of rotten eggs, is produced in coal pits, gas wells, sulfur springs, and from decaying organic matter containing sulfur. Controlling emissions of this gas, particularly from municipal sewage treatment plants, has long been considered desirable. More recently, protecting electronic apparatus from the corrosive fumes of these compounds has become increasingly important. H.sub.2 S is also flammable.
Ammonia (NH.sub.3), also a colorless gas, possesses a distinctive, pungent odor and is a corrosive, alkaline gas. The gas is produced in animal rooms and nurseries and its control also has long been considered desirable.
Chlorine (Cl.sub.2) is a greenish-yellow gas with a suffocating odor. The compound is used for bleaching fabrics, purifying water, treating iron, and other uses. Control of this powerful irritant is most desirable for the well-being of those who work with it or are otherwise exposed to it. At lower levels, in combination with moisture, chlorine has a corrosive effect on electronic circuitry, stainless steel and the like.
Formaldehyde (HCHO) is a colorless gas with a pungent suffocating odor. It is present in hospital morgues, and because it is intensely irritating to mucous membranes, its control is desirable.
Urea (CH.sub.4 N.sub.2 O) is present in toilet exhaust and is used extensively in the paper industry to soften cellulose. Its odor makes control of this compound desirable.
Carbon monoxide (CO), an odorless, colorless, toxic gas, is present in compressed breathing air. Oxygenation requirements for certain atmospheres, including those inhabited by humans, mandate its control.
Oxides of nitrogen, including nitrogen dioxide (NO.sub.2) nitric oxide (NO), and nitrous oxide (N.sub.2 O), are compounds with differing characteristics and levels of danger to humans, with nitrous oxide being the least irritating oxide. Nitrogen dioxide, however, is a deadly poison. Control of pollution resulting from any of these oxides is desirable or necessary, depending on the oxide.
Mercaptans and amines, including methyl mercaptan (CH.sub.3 SH), butyl mercaptan (C.sub.4 H.sub.9 SH) and methyl amine (CH.sub.5 N), are undesirable gases present in sewerage odor. The control of these gases is desired for odor control.
Ethylene (C.sub.2 H.sub.4) is a colorless, flammable gas that is a simple asphyxiant which accelerates the maturation or decomposition of fruits, vegetables, and flowers. Control of this compound prolongs the marketable life of such items.
Attempts have been made to provide solid filtration media for removing the undesirable compounds described above. Desired features of such media are a high total adsorption capacity for the targeted compound, high efficiency in removing the compound from an air stream flowing over the media, and a low ignition temperature (non-flammability). For example, U.S. Pat. No. 3,049,399 describes a solid oxidizing system in pellet form composed of activated alumina, Al.sub.2 O.sub.3, impregnated with potassium permanganate, KMnO.sub.4. This pellet provides air purification and odor control by both adsorbing and adsorbing odors, and then destroying the collected odors by the potassium permanganate's controlled oxidizing action.
The prior art reveals that activated carbon will physically adsorb considerable quantities of hydrogen sulfide. See, for example, U.S. Pat. No. 2,967,587. See also French Patent No. 1,443,080, which describes adsorption of hydrogen sulfide directly by activated carbon, which is then regenerated by hot inert gas or superheated steam.
The prior art also reveals that better removal of sulfur compounds can be accomplished by the catalysis of the oxidation of hydrogen sulfide to sulfur, based on the ability of carbon to oxidize hydrogen sulfide to elemental sulfur in the presence of oxygen. Ammonia may be added to an influent gas stream of hydrogen sulfide and oxygen to provide catalysis. Silicate-impregnated activated carbon is also effective. The residual adsorbate, however, may not be removed by extraction with alkaline solutions. See South African Patent No. 70/4611. Treatment with a 1% solution of NaOH restores the adsorption capacity of activated carbons used for adsorption removal of hydrogen sulfide gas. Boki, Shikoku Igaku Zasshi, 30(c), 121-8 (1974) (Chemical Abstracts, Vol. 81.
See also, for example, French Patent No. 1,388,453, which describes activated carbon granules impregnated with 1% iodine (I.sub.2) for this use. South African Patent No. 70/4611 discloses the use of silicate-impregnated activated carbon. Swinarski et al, Chem. Stosowana, Ser. A 9(3), 287-94(1965), (Chemical Abstracts, Vol. 64, 1379c), describe the use of activated carbon treated with potassium salts, including potassium hydroxide (KOH) for hydrogen sulfide adsorption. Activated carbon has also been impregnated with a solution of sodium hydroxide (NaOH) and potassium iodide (KI).
Other known methods of impregnating activated carbon for removing sulfur compounds from gas streams include the use of carbonate or hydroxide of potassium or sodium as the impregnate. See Japanese Patent Application No. 39-23720. Another method removes mercaptans from exhaust gas by contact with an adsorbent impregnated with a liquid mixture of an alkaline material. U.S. Pat. No. 3,391,988. Subsequent patents have taught different treatments of activated carbon with NaOH and, optionally, lead acetate (PbOAc), and have indicated the influence of the chemical reaction therein combined with the physical adsorption of the activated carbon. See U.S. Pat. No. 4,072,479 and U.S. Pat. No. 4,072,480. U.S. Pat. No. 4,072,479 suggests that hydrogen sulfide is oxidized to elemental sulfur in the presence of activated carbon, and that the presence of moisture on the activated carbon is significant. We cannot confirm the accuracy of these observations. Another method for removing sulfur and other compounds from gas streams utilizes a product known as Purakol K, which is carbon impregnated with NaOH and KI.
Other uses of impregnated carbon include removing water from air (dessication), see, for example, Soviet Union Patent No. 1,219,122 (activated carbon combined with aluminum oxide; a binder, calcium hydroxide; and lithium bromide); and the removal of acidic contaminants from gas streams, see, for example, U.S. Pat. No. 4,215,096 (activated carbon impregnated with sodium hydroxide and moisture, for the removal of chlorine from gas streams) and U.S. Pat. No. 4,273,751 (activated carbon impregnated with sodium hydroxide and moisture, for the removal of sulfur oxide gases and vapors from gas streams).
Japanese Patent No. 61-178809 teaches water purification by treatment with activated carbon loaded with metallic copper or copper salts. Several patents teach alumina and carbon adsorbents, including U.S. Pat. No. 3,360,134 (alumina hydrate contacted with a carbonaceous solution; used as a decolorizing agent, a reviving agent for precious metal electroplating bath for the removal of constituents from cigarette smoke, and as an adsorbent in pressure or gravity flow percolation beds); U.S. Pat. No. 4,449,208 (powdered carbon, dense alumina, and a binder, for increasing heat capacity of the adsorbent to enhance the operation of adiabatic pressure swing adsorption processes by decreasing the cyclic temperature change in the adsorbent bed during each cycle of the process); U.S. Pat. No. 3,819,532 (ground graphite and finely divided alumina adsorbent, for removing aromatics, heterocyclics, sulfur compounds, and colored materials from lubricating oils); and U.S. Pat. No. 3,842,014 (ground graphite and alumina binder, for adsorbing parafins). Such art generally teaches a substrate consisting primarily of activated carbon with a relatively small amount of alumina.
None of the prior art compositions have effectively solved the problem of the combustibility of activated carbon. This problem can be critical in installations such as nuclear power plants.
With respect to the use of alumina substrates as compositions for deodorizing air, the prior art teaches that alumina is one substrate which is suitable for chemical treatment with potassium permanganate. It appears that the permanganate destroys odors by oxidation: EQU MnO.sub.4.sup.- +8H.sup.+ +5e.sup.- .fwdarw.Mn.sup.+2 +4H.sub.2 O (acid) EQU 3MnO.sub.4 +2H.sub.2 O.fwdarw.MnO.sub.2 +2MnO.sub.4 +4OH (alkaline) EQU MnO.sub.4.sup.- +2H.sub.2 O+3e.sup.- .fwdarw.MnO.sub.2 +2H.sub.2 O (neutral)
Because the permanganate will not ionize to release the active permanganate ion unless water is present, the substrate must be hydrophilic and the reaction must take place in normal ambient humidity. See U.S. Pat. No. 3,049,399. Other art teaches a method of producing granular activated alumina uniformly impregnated with a solid oxidizing agent for use in treating fluid streams. See U.S. Pat. No. 3,226,332, the contents of which are incorporated herein by reference.